There is general agreement that periodontal disease and much of its symptomatology eventuates as a consequence of the microbial flora present in periodontal pockets and in gingivitis. There is less agreement on the role of supragingival plaque in the etiology of the disease. Due to the extremely complex and diverse microbial population of the pocket and the huge numbers of organisms present in subgingival plaque, it is inevitable that microbial interactions, such as synergism and antagonism, will occur. The literature of such interaction is relatively sparse. Since loss of teeth, particularly in human populations above 30 years of age, occurs primarily, in the vast majority of instances, as a consequence of periodontitis, the health relatedness of research on the presumed inciting agents is obvious. The subgingival flora is composed to a large extent of Gram-negative obligate and facultative anaerobes. Various genera such as Bacteroides, Fusobacterium, Wolinella, Eikenella, Actinobacillus, Treponema and Selenomonas as well as the Gram-positive Streptococcus, Actinomyces and Eubacterium (in addition to others) are among the organisms potentially interacting in periodontitis. By means of a U tube, the arms separated by a bacterial filter, by "gutterplates" and by spotting, it is proposed to study in vitro the effects of each of these organisms on the others, in both pure and mixed culture. In addition, supragingival and subgingival plaque will be collected; in the case of the latter by curettage and by paperpoints for non-attached members of the flora. The plaque will be tested for effects in low and high concentrations on pure and mixed cultures of the above listed organisms in low and high concentrations. Conversely, low and high concentrations of the above organisms will be tested for their effects on microbial members of the plaque.